Spillway + Internal Weir

I am modeling a tributary of the Mississippi River using W2. There are dikes to control flow in-between the Mississippi and the tribs. In most of the time, the elevations in the Mississippi is higher than the tribs. When the elevation is higher than the dike, water flows into the tribs from the Mississippi. Some times, the elevations in the tribs may get higher than the dike (during storms) and water flows into the Mississippi. Othertimes, the both the elevations in the Mississppi and tribs are higher than the dike and water flow freely back and forth. At first, I am using Internal Weir to configure these dikes and found it didn't work because Internal Weir calculation only set horizontal velocity to 0 after solving the equation (Am I right?). So it does not block water when the elevations are lower than the dike. If I use spillway, I need to know the flow rates when water flows freely back and forth at the time when water elevations are way above the dike elevations, which are supposed to be calculated by the model. Are there other ways to solve the problem? Thanks a lot!